1. Field of the Invention
The present invention relates to the field of delta-sigma modulators. In particular, the invention relates to method and apparatus for adjusting phase and offset for multiplexed and non-multiplexed delta-sigma modulators.
2. Background Art
One of the fundamental factors in the performance of digital signal processing systems is the precision of analog to digital conversion. Precision analog to digital conversion has application in fields such as speech-processing, ISDN, digital audio tapes, digital radio, modems, compact disc players, sonar signal processing, and other fields involving signal acquisition and processing. In many of these applications the performance of an analog to digital converter represents the fundamental limit for the operation of the system as a whole. Recently, "delta-sigma modulators" have received considerable attention as candidates for high-resolution analog to digital conversion. Delta-sigma modulators offer high resolution without the component matching required for successive-approximation type analog to digital converters or the restricted speed of integrating converters. Moreover, delta-sigma analog to digital converters fit MOS fabrication trends: they consists largely of digital circuitry, and can therefore scale efficiently in size as digital MOS circuit sizes shrink.
FIG. 1 shows a block diagram of a typical prior art "delta-sigma modulator" 10. As shown in FIG. 1, the prior art delta-sigma modulator consists of integrator 23, analog to digital converter (quantizer) 25, and a digital to analog converter 20 in a feedback loop. An analog input signal x(t) 12 is supplied to the input of delta-sigma modulator 10. Analog input x(t) is sampled at a sampling frequency Fs (14). The sampling frequency Fs is well above the "Nyquist frequency." The Nyquist frequency is defined as a sampling frequency which is twice the highest frequency of the frequency components present in input signal x(t). Digital filter 30 is coupled to the output of quantizer 25 and functions as "decimator" 31. "Decimation" is the name given to processes that lower the sample rate of digitally encoded signals which are sampled at a frequency above their Nyquist rate. In order to prevent out of band signals from aliasing into the baseband of interest, decimation is performed only after the out of band signals are adequately attenuated by digital filter 30. If some of the quantization noise generated by the delta-sigma modulator at y(nT) 28 is in the frequency range that is not of interest, then the resolution of the analog to digital converter can be increased with filtering and decimation. A digital decimation filter can be realized in several ways. For example, FIR filters have been used as decimation filters in delta-sigma modulators.
Analog to digital conversion is a process that necessarily introduces errors into a signal due to quantization. The difference between the output of an otherwise perfect converter (or "quantizer") and that which is expected of a converter with unlimited resolution is called "quantization noise." The level of this "quantization noise" is reduced in converters of higher resolution with finer quantization levels, but nonetheless it remains non-zero. If the analog input is sufficiently random, the spectrum of the quantization noise can be approximated as white, with its energy spectrum equally distributed between zero and Fs, where Fs is the sampling rate. The effective resolution of a converter can be increased by filtering its output and thereby reducing the level of the quantization noise. A commensurate reduction in the available signal bandwidth is a consequence of the filtering, but may be acceptable if the sampling and conversion rates are high. The sampling and conversion of a signal at a rate much higher than the signal frequency is a technique termed oversampling. The oversampling ratio is the ratio of the actual sampling rate to the Nyquist rate.
In operation, the prior art delta-sigma modulator 10 receives an analog input signal denoted as x(t) 12. Input signal x(t) is sampled at a rate much higher than the Nyquist rate. The oversampling of input signal x(t) results in a high resolution of the final output 32 which is typically converted to the much lower Nyquist rate with the aid of decimator 31. Output y(nT) 28 is a one-bit output which is a pulse density representation of the input x(t). The one-bit output y(nT) is digitally filtered by decimator 31 to reduce the high frequency quantization noise produced by quantizer 25 (while leaving the base band unaffected). Digital to analog converter 20 is also a one-bit converter. The output of digital to analog converter is subtracted from the analog input signal x(t). This results in an error signal which is integrated by integrator 23. The resulting signal is "quantized" (i.e. converted) by one-bit analog to digital converter 26. The one-bit quantization performed by modulator 10 generates a high level of quantization noise.. This quantization noise is spectrally shaped by the integrator and feedback loop of the modulator such that most of the noise energy at output 32 lies at high frequencies outside the baseband (i.e., the frequency band of interest).
Although delta-sigma modulators do not typically have good response at DC, when averaged over a large number of outputs they can have excellent theoretical low frequency response. The prior art delta-sigma modulators have a disadvantage when the DC value of an input must be measured. Unwanted offsets present within the delta-sigma modulator can not be distinguished from the DC value of the input.
Analog techniques for correcting and reducing offsets (chopper stabilization, etc.) are typically very difficult to implement and have shown limited success. Another method of offset correction averages (filters) the output over a long period of time and subtracts this value from the input. While this is effective at removing the DC component it precludes the measurement of the DC value. This is because it removes both the DC offset of the modulator and the DC value of the signal.
Phase shifting a signal is desirable in many applications. For a given frequency, a delay in time will correspond to a particular phase shift. At the output of the FIR filter 32 the time interval between samples has been increased due to decimation. By time shifting at the output of the delta-sigma modulator (y(nT) 28) a fine adjustment in the phase of a signal can be made with simple delay elements (flip-flops).
This is adequate for some applications, however a problem arises when a single delta-sigma modulator is multiplexed among multiple input channels and each channel requires different phase adjust. Cross talk between successive channels results whenever phase shifts between successive channels are not equal. This is because data stored in the phase adjusting flip-flops from a previous channel will be used for the present channel.
The present invention overcomes the disadvantages of the prior art delta-sigma modulators. The invention results in DC offset and fine phase correction for delta-sigma modulators. The invention also results in a simple and low cost method for DC offset and fine phase correction. Accordingly, the invention allows use of delta-sigma modulators in precision DC applications where small DC offsets are required.